Of semiconductor lasers, there is a VCSEL (Vertical-Cavity Surface-Emitting Laser) which makes light to be resonated in a direction perpendicular to a substrate surface, and emitted in the direction perpendicular to the substrate surface, other than a type which makes light to be resonated in a direction parallel with the substrate surface, and emitted in the direction. The VCSEL has many advantages. For example, the power consumption of a system with the VCSEL is small, it is possible to perform high-speed modulation even when the current is low, and the temperature controller can be simplified since the variation of the characteristic in response to temperature change is small. The VCSEL is widely applicable in fields such as a light source of a Gigabit Ethernet (registered trademark) or a Fiber Channel, a laser printer, and an optical interconnect.
On the other hand, there are problems that the VCSEL may be damaged due to ESD (electrostatic discharge), and the fault that the electrooptical property of the VCSEL deteriorates tends to occur. Thus, various techniques of detecting the occurrence of the damage and the fault due to the ESD are proposed. For example, a technique is proposed in which a leak current when a VCSEL is reversely biased is tested as a means to determine whether the VCSEL is damaged due to the ESD.